DNA repair in mitotically growing cells of yeast is under the control of more than 20 genetic loci. Damage (primarily pyrimidine dimers) produced by UV-light can be repaired through three pathways. We have developed a method for identifying and analyzing the repair of pyrimidine dimers by these pathways after extremely low doses corresponding to only 1-2 dimers per chromosome. Chromosomal size DNA obtained through gentle lysis of cells is treated with Micrococcus luteus extract which recognizes and nicks the DNA at pyrimidine dimers. The DNA is subsequently analyzed using sucrose gradient techniques. This method has enabled us to identify low levels of repair (leaky) in previously identified repair deficient mutants and to demonstrate that post-replication repair in yeast does not occur via a recombinational mechanism.